In some cases the bandwidth available on a network facility cannot accommodate the bandwidth required to carry a service. In such cases virtual concatenation (VCAT) is used to transport a larger service by splitting the payload into several members that may travel toward a destination node over different routes in the network.
When the members of a given virtual concatenation group travel over different routes, the inherent transmission latency will introduce a differential delay between the members at the destination point. The differential delay is due primarily to the difference in distance in each route. For example, an optical route typically introduces a 5 ms latency per 1000 km. To accommodate a differential delay compensation is required at the destination node that will re-combine the members of the virtual concatenation group. The current approach is to buffer the members and then align them for proper re-combination. Although currently existing systems accommodate a theoretical 256 ms inter-member differential delay, the practical limit is likely less due to the large amount of memory required. If the differential delay exceeds the buffering capability of the destination node, the signal is impacted.
When a service is virtually concatenation and its members are routed over different facilities in the network, it is likely that those facilities are protected. In the event of a protection switch a member may be re-routed over a new route that may affect the distance the signal travels to the destination node and this has a direct impact on the latency of the signal transmission. If the re-routing brings the inter-member differential delay at the destination point beyond what the buffering hardware can handle, the signal is lost.
Against this background, it clearly appears that a need exists in the industry to provide improved mechanisms in a network transporting Virtual concatenation groups to control the latency of the individual members of the group.